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320                              CHEMICAL ENGINEERING
suspended a fine wire forming the other electrode. The dust-laden air passes up through or occasionally down through the batteries of pipes forming one of the sets of electrodes and on coming within the influence of the high-tension current the dust particles are immediately more or less completely knocked down. The moment the particles are electrified they are attracted to the pipe or collecting electrode and by the use of an interrupter in the current circuit the dust particles will be able to work their way down into the hoppers into which the pipes are inserted. The Cottrell process is particularly valuable with fumes which would attack cotton or wool in cloth nitration and most of its first installations were around copper furnaces and converters the fumes from which containing large amounts of S03 could not be successfully filtered through cloth.
The great disadvantage of the process has been its large installation cost. It is estimated that the cost of installing a Cottrell dust-collecting system, together with leaching and evaporating systems for a byproduct potash plant from a cement mill of 1,000,000 bbl. annual capacity would cost about $150,000. * The usual range of K20 in clay or shale suitable for portland cement is from 0.8 to 1.25 per cent. Amounts as high as 2K per cent are occasionally found. A 100 ft. rotary kiln will produce about 4 to 7 tons of dust daily with potash content from 4 to 10 per cent. If soils require lime the collected dust may sometimes be used as a fertilizer without further preparation. Potash recovery from cement rests upon the observation of Dr. W. F. Hildebrand that after burning the raw mixture the greater portion of the potash and soda had been expelled by volatilization. Of the amount volatilized together with the dust from the kilns the Cottrell process will throw down from 50 to 80 per cent. The slow solution of the potash following the Cottrell process is due in part to the formation of an insoluble double sulphate of potassium and calcium. By heating the dissolving water to 85C. and higher and maintaining these temperatures during the subsequent filtering operations the greater part of the soluble potash can be recovered.
In fume recovery2 the Cottrell process gave 100 per cent precipitation from copper furnaces containing large amounts of SOs. The proportion of SOa to SO2 ranged from 1 : 18 to 1 : 8. It was necessary to reduce the temperature of the fumes to about 90C. to get the best results. The velocity of the gases in the electrode pipes had to be less than 15 ft. per second. Water was also necessary to give good results, 3 to 5 per cent of the weight of dust collected. When there was much SOs the electrode pipes became non-conductors and water tended to overcome this difficulty. The fume recovered contained 41 per cent lead as basic sulphate.3
One curious result has been recorded by Howard: during the progress of the test work with fumes from various furnaces it was observed that during the blow on white metal, where a conductive deposit was obtained in the pipes practically all the lead was precipitated at a temperature of 340C. but the S03 and the As203 escaped. To precipitate the latter and obtain complete smoke clearance it was necessary to reduce the temperature to about 90C. At a certain western lead blast-furnace plant fumes
1 Of. "Potash in 1917, Min. Res. of the U. S., 1917," Part II; J. J. PORTER, "The Recovery of Potash as a Byproduct in the Manufacture of Cement," presented at Fall Meeting of Portland Cement Association, Chicago, Sept.. 1917, and J. TREANOR, "The Experience of the Riverside Portland Cement Co. in the Recovery of Potash from Cement Flue Dust," presented at the December meeting, Portland Cement Association, 1917.
2  Cf.  "Electrical Fume Precipitation at Garfield," by W. H. HOWARD, Trans. A. I. M. E., XLIX, p. 540.
3  Spray nozzles of the Schiitte-Koerting type can be used advantageously for moistening fume. EDITOK.